1. Field of the Invention
The present invention relates to a method and a device for distance measurement by pulse radar that accurately measures the distance of the device to a target based on the reflected pulse signals that is a high-frequency signal sent from the device and reflected by the target.
2. Description of the Related Art
Generally, pulse radars transmit pulses that are modulated and have the designated frequency (76 GHz for car mounted radar, for example), and measure the distance of the radar to a target by calculating the lag time of the reflected pulse signal from the target.
FIG. 1A shows the pulse signal sent to four targets and its reflected pulse signals. Each chart shows (1) the single pulse signal, (2) the received signal, (3) the repeating pulse signal, and (4) the received signal.
Here, the received signal A1, B1, C1 and D1 are the reflected signals of the pulse signal S1 measured by the receiving unit of the pulse radar, and expressed by its intensity. The reflected signals are from the targets (target A, B, C and D), and each of those has a different distance from the signal sending point. Similarly, the received signal A2, B2, C2 and D2 are the reflected signals of the pulse signal S2 from the targets, and are expressed by their intensity.
As FIG. 1 (1) shows, when the single pulse signal S1 is transmitted, the reflected signals are returned from the targets detecting received signals A1, B1, C1 and D1. Here, the lag time of the received signal C1 and D1 from the point of the pulse signal generation is addressed as lag time τ1 and lag time τ2, respectively.
Meanwhile, as FIG. 1A (3) shows, when the repeating pulse signals with cycle T, a pulse cycle, is transmitted to the targets. Signals A1, B1, and C1 of the pulse signal S1 are detected within the same pulse cycle of the pulse signal S1. However, because lag time τ2 of the received signal D1 is longer than the pulse cycle T, received signal D1 is detected as a receiving signal for the pulse signal S2. (The received signals like D1 in this example are hereafter addressed as ghost signals).
Accordingly, there is a limit that when the generation cycle of the pulse signal is τ, and the longest measurable distance is L, generation cycle T of the pulse signal τ has to be longer than 2L/c (c is the speed of light), The time that a pulse signal is reflected by a target and returns to where it was transmitted. This is called the uncertainty of distance measurement by the repeating pulse.
Laser Processing Technology (Matsuo Sekine, IEICE), In order to eliminate the uncertainty of distance measurement by a repeating pulse, describes a pulse radar that removes echo utilizing the characteristic of the reflected pulse signal, that is, the farther the target is, the smaller the intensity of its reflection becomes.
Also, Japanese Publication Unexamined Application No. Showa61-133885 discloses a method to send a mixed pulse. A plurality of short pulses with different repeating cycles are mixed in one cycle of the long pulse repeating cycles and to eliminate the signals of the short pulse that appears at the different time points in every cycle where any of the sending pulse timing is referred as the reference pulse timing.
Japanese Publication Unexamined Application No. 2000-111639 disclosed a method to detect the target in a range that would correspond to the uncertainty of distance measurement; that is a round-trip of the distance requires a longer time period than the cycle of pulse generation by simultaneously generating a plurality of signals with different frequencies and detecting phase with N number of detectors.
However, the pulse radars for short distance measurement with Laser Processing Technology has a problem that ghost signals cannot be securely eliminated by separating reflection intensities. The process requires a short cycle of pulse generation to secure enough separation accuracy (resolution). This is a problem when the size and the reflection intensity of the targets differ greatly such as a car and a person, and especially when high accuracy is required for measurement of short distances.
Also, the disclosed method in Japanese Publication Unexamined Application No. Showa61-133885 only eliminates the intervention between pulses in long and short compound pulse radar; the method is not for the elimination of the echo (ghost signal) that exceeded the time cycle of pulse repetition.
The method in Japanese Publication Unexamined Application No. 2000-111639 requires a large structure for the device to send out a plurality of signals simultaneously and a phase detector to measure the distance. As explained above, it was difficult for a short pulse radar which enables highly accurate measurement over the distance range of about 10 cm to over 10 m to separate the ghost signals, that is reflected pulses from an object which is located beyond the detection distance limit for a repetition period, when pulse frequency repetition is increased to improve S/N (the ratio of signal to noise) of the received signal as it is shown in FIG. 1A. In addition, there are some cases that sending and receiving signal intervention, that is the pulse radar receives its own sending signal as receiving signal by mistake when the radar sends out a pulse signal, prevents the periodical detection of reflected signal.
Moreover, like the existing pulse radar for weather observation the use of long period pulses greatly increases the required electrical power at the point of pulse transmission in order to improve S/N, and causes problems in cost and circuit production.